1. Field of the Invention
This invention concerns a high density information recording carrier for use in an optical recording and reproducing apparatus for recording and reproduction of information by the use of laser beams. More specifically, it relates to a high density information recording carrier made of plastic having excellent durability and further to a high density information recording carrier for magneto-optic recording and reproduction excellent in magneto-optic characteristic.
2. Description of the Prior Art
Optical information recording carriers include those discs exclusively used for reproduction such as video discs and digital audio discs, so called DRAW discs which are capable of writing, and further so called E-DRAW discs which are capable of writing, erasing and writing such as magneto-optic discs. They are produced or manufactured by a method of preparing a stamper from an original disc engraved with signals or guide tracks, molding a copy disc made of plastic from the stamper to obtain a plastic substrate having a surface containing signals or guide tracks, and forming various types of functional membranes thereon. The signals contained in the recording surface comprise those bits corresponding to digital signals or analog signals in the case of read-only memory discs, and bits and guide tracks corresponding to signals for use in addressing and error correction in the case of above-mentioned DRAW and E-DRAW discs. Each bit has a width of 0.5-0.8.mu., a length of 0.5-2.0.mu. and a depth of 0.06-1.2.mu. and each guide groove has, respectively, 0.5-1.0.mu. width and 0.06-1.2.mu. depth. They are disposed in a single or multiple stages. As a functional membrane, in the case of the read-only memory discs, a reflecting metal membrane, such as aluminum, is coated at the recording surface of the plastic substrate by the method of vapor depositioning, sputtering or ion-plating and in the case of DRAW discs, aluminum, chromium, lead, gold, rhodium, zinc, copper, antimony, tellurium, indium, bismuth, carbon disulfide - tellurium, tellurium carbide, tellurium arsenide, thin pigment membrane, silver plus polymer or the like is coated as a recording medium. Further, in the case of the E-DRAW discs, phase transformable material such as tellurium oxide, a magnetic membrane made of manganese--bismuth, gadolinium--cobalt, iron--terbium, iron--terbium--gadolinium, or iron--terbium--cobalt, or a semiconductor membrane such as of arsenic--selenium--sulfur--germanium is coated on a plastic substrate by the method of vapor depositioning, sputtering, ion-plating or the like. Usually metal, for the purpose of reflection and/or inorganic material such as silicon oxide or polymeric material such as of acryl or epoxy, for the purpose of protection, is often coated further on the above-mentioned metal material.
Generally, the above-mentioned functional membranes are usually formed on a glass substrate by the method of sputtering or vapor deposition. Such a glass material is used as the substrate, because it exhibits:
(1) high optical transmittance,
(2) high heat conduction and low heat expansion coefficient.
(3) no hygroscopic deformation at all, and
(4) good bondability with the recording layer and others.
However, it is difficult to machine, heavy, easy to crack, difficult to provide with guide tracks and poor in mass productivity.
On the other hand, plastics such as polymethylemethacrylate and polycarbonate are preferable for the substrate for information recording discs with their optical transmittance of about 90 percent and can be produced as inexpensive substrates in a great quantity by the method of injection molding or the like.
However, these plastic substrates are relatively soft, have a considerable heat expansion coefficient and, in addition, exhibit relatively large hygroscopic deformation. Particularly, the hygroscopic deformation results in a serious problem in the optical recording and reproduction of information. Further, owing to gas permeability, it is inevitable that the recording layer will deteriorate by oxidation due to oxygen permeating through the substrate. In order to eliminate the above drawbacks, organic silanes are coated or oxides such as SiO.sub.2 are formed through sputtering or the like on the substrate; however, none of them has been quite satisfactory. Because it is necessary for the organic silanes to be immersed or spin coated by using a solvent and, thereafter, cured at about 90.degree. C., it causes deformation to the plastic substrate. While on the other hand, formation of inorganic oxides such as SiO.sub.2 through sputtering brings about the generation of pin holes which make it difficult to completely defeat the introduction of moisture and oxygen.